Method for inducing resistance to embrittlement by neutron irradiation and products formed thereby

ABSTRACT

Metals and alloys containing incidental amounts of boron are doped with a boride-forming element in an amount effective to induce formation of an intragranularly dispersed precipitated phase at elevated temperatures to produce a product having enhanced resistance to radiation-induced embrittlement at high temperatures.

United States Patent [191 Martin et al.

[ Apr. 16, 1974 i 22 Filed:-

.[ METHOD FOR INDUCING RESISTANCE TO EMBRITTLEMENT BY NEUTRON IRRADIATION AND PRODUCTS FORMED THEREBY [75] Inventors: William R. Martin; James R. Weir,

both of Oak Ridge, Tenn.

[73] Assignee: The United States of America as represented by the United States Atomic Energy Commission, Washington, D.C.

Aug. 8, 1972 [2]] Appl. No.: 278,857

Related US. Application Data [60] Continuation-impart of Ser. No. 47,930, June 6, 1970, abandoned, which is a division of Ser. No. 603,085, Dec. 15, 1960, abandoned, which is a continuation of Ser. No. 506,914, Nov. 5, 1965, Pat.

[56] References Cited UNITED STATES PATENTS 2,921,850 l/l960 lnouye et a1. 75/171 3,479,157 11/1969 Richards et a1. 75/128 F 3,301,668 l/1967 Cope 75/128 3,347,715 10/1967 148/12 3,573,109 3/1971 148/12 3,440,037 4/1969 Martin et a]. 75/128 R 3,576,622 4/1971 McCoy, Jr. 75/171 FOREIGN PATENTS 0R APPLICATIONS 646,413 4/1964 Belgium 148/12 Primary Examiner-Charles N. Lovell Attorney, Agent, or Firm-John A. Horan; David S. Zachry; Irving Barrack [5 7] ABSTRACT Metals and alloys containing incidental amounts of boron are doped with a boride-forming element in an amount effective to induce formation of an intragranularly dispersed precipitated phase at elevated temperatures to produce a product having enhanced resistance to radiation-induced embrittlement at high temperatures.

2 Claims, No Drawings METHOD FOR INDUCING RESISTANCE TO EMBRITTLEMENT BY NEUTRON IRRADIATION AND PRODUCTS FORMED THEREBY This application is a continuation-in-part of our copending application Ser. No. 47,930, filed June 6, 1970, now abandoned, which was in turn a division of application Ser. No. 603,085, filed Dec. 15, 1966, now abandoned and which in turn was a continuation in part of application Ser. No. 506,9l4,filed Nov. 5, 1965, now U.S. Pat. No. 3,440,037.

The invention described herein was made in the' course of, or under, a contract with the HS. Atomic Energy Commission.

BACKGROUND OF THE INVENTION When certain metals and alloys containing incidental quantities of boron, such as nickel and iron-base alloys, are exposed to neutron environments at elevated temperatures, i.e.,. in excess of about 500C, a nonrestorable embrittlement occurs. This form of embrittlement is distinguishable from embrittlement encountered at lower temperatures. Below a threshold temperature characteristic of a given metal or alloy, a reduction in ductility takes place on'exposure to neutron irradiation which is attributed to atomic displacement. Loss in ductility under such conditions does not affect fracture in the metal or alloy. On the other hand, loss in ductility resulting from neutron exposures above the characteristic threshold temperature results in extensive intergranular fracture which, in turn, leads to a severe reduction in the useful life of the material. lts primary effect is to produce a serious loss in creep ductility, and hence, loss of stress rupture life at high temperatures.

The general behavior of metals and alloys irradiated at high temperatures is characterized by fracture at strains at which grain boundary cracks in unirradiated alloys are substantially absent. Once a grain boundary crack forms it propagates at an extremely rapid rate. The integrity of structural components and fuel elements of a nuclear reactor can depend on factors that are affected by or related to the ductility of the material such as creep rupture life.

Helium-induced embrittlement has been recognized to occur in such metals as beryllium, copper, iron, and nickel, as well as alloys containing these metals as a major alloying ingredient. The problem is that elemental boron in solidsolution and helium, a nuclear transmutation product, are both horophillic elements. That is to say, they have a tendency to diffuse or segregate to grain boundaries. When the boron in the grain boundaries is transmuted to helium, the helium, being virtually insoluble in the metal or alloy, forms bubbles within the volume of the metal or alloy and serves as grain boundary crack nuclei.

SUMMARY AND INVENTIVE CONCEPT The problem then, and the main object of. this invention, is to provide a process and product means to prevent or reduce the horophillic, i.e., intergranular boundary-seeking tendencies of boron and helium in the solid state medium of a metal or alloy.

The inventive concept of this invention is predicated on the formation of a homogeneously dispersed stable intragranular boron or boride-containing phase or compound in a matrix of a metal or alloy in which the boron exists in incidental amounts. By an incidental amount" is meant a concentration of boron which has not been deliberately added to a metal or alloy to provide a specific metallurgical denotion, but which exists on an impurity level up to as much as ppm on a weight basis.

While the absolute amount of such boron impurities is quite small, the ratio of its mechanical effect after nuclear transmutation to helium over its absolute concentration is enormous. Thus, it has been found that in a stainless steel alloy containing from 0.01 to about 100 ppm helium, as the deformation temperature is increased the amount of helium, and hence boron needed to initiate embrittlement decreases at an exceedingly rapid rate.

The formation of a homogeneously distributed insoluble phase or compound in a metal or alloy of the defined class will reduce the propensity of helium generated by the transmutation of boron-l0 to migrate or segregate to the intergranular boundaries. Instead, the stabilization of boron as an insoluble boride will provide, in effect, a substitute boundary to satisfy the boundary-seeking propensities of helium as well as boron. Stated in other terms, the intragranular precipitate-matrix interfaces serve as traps or sinks for any helium generated from the nuclear transmutation resulting' from the interaction of boron with thermal neutrons as well as interaction of fast neutrons with elements other than boron. Thus, since the inventive concept involves the formation of stable, homogeneously dispersed intragranular precipitated phase, it results in, and it is a further object of this invention to provide, product and process means for obtaining a metal or alloy with incidental quantities of boron or other source of helium, characterized in having a reduced susceptibility to high temperature embrittlement in thermal as well as fast neutron environments.

DESCRIPTION With the foregoing discussion of the background and inventive concept in mind, the process and product aspects of the invention will now be described.

As a process, the present invention comprises adding a boride-forming element to a metal or alloy containing incidental amounts of boron-10 and designed for service in a neutron environment at elevated temperatures, heat treating the resultant alloy in the solid state at a temperature above the intended service temperature for a time sufficient to form a substantially homogeneous dispersion of an intragranular precipitate. In the context of the present invention, a boride-forming element is defined as at least one selected from the group consisting of titanium, zirconium, and hafnium at a concentration sufficient to induce formation of an intragranular precipitate at a heat treatment above the intended service temperature of the resultant alloy.

Having described the inventive concept and modus operandi in general terms, reference will now be made to a number of specific embodiments of the process and product aspects of the aforementioned inventive concept.

EXAMPLE I This example is described in two parts. In the first part (A), the presence of helium in a type 304 stainless steel is shown to cause a loss of high-temperature ductility. In the second part (B), the problem having been experimentally verified in (A), means are now demonstrated to overcome the problem. PART A v The composition of the steel was in weight per cent C, 0.046; Mn, 1.77; P, 0.023; S, 0.10; Si, 0.62; Cr, 18.15; Ni, 9.60; Co, 0.25; Mo, 0.33; and the balance iron. It was hot rolled from 0.060-inch thick sheet to 0.040-inch thick strip, cold rolled to 0.020-inch thick strip, annealed 1 hour at 1,038C in argon, and tensile samples of ii-inch gage length, 19-inch width prepared. A final l-hour anneal at 1,038C in argon was followed by cooling in the cold zone of the furnace.

Five samples were bombarded in a cyclotron in the following manner: a-particles accelerated to 8010.5 Mev were moderated to distribute the helium uniformly through the sheet thickness. The temperature of the samples at no time exceeded 200C. The samples were irradiated to approximately 0.1 ppm (atom fraction) of helium, calculated from the total beam current and known losses.

Five other samples were irradiated without the moderator, allowing equivalent or more damage to the samples, but also permitting a-particles to pass completely through the samples.

These samples were tensile tested at a strain rate of 25 0.026 in/in/min in air over the temperature range from 500 to 900C in 100 increments. A minimum of three unirradiated control samples was also tested under the same conditions. The data from these tests are tabulated in T b e .3..

The results indicate no change in the strength properties of the steel as a result of a-bombardment with and without the moderator in place. This is not surprising,

since displacement damage in type 304 stainless steels may recover below 500C. The uniform and total elongations of the samples bombarded without the moderator were sequentially not affected by the bombardment, nor were the uniform elongations of the samples bombarded with the moderator in place altered.

However, the total elongations of the samples containing helium, irradiated with the moderator in place, were decreased substantially over the temperature range from 700 to 900C, where neutron irradiation causes embrittlement of stainless steel. This demonstrates that the presence of helium is responsible for the high-temperature embrittlement of stainless steel and illustrates the problem of the invention.

PART B A heat of type 304 stainless steel similar in composition to the 304 stainless steel of Part (A) but modified with 0.2 percent titanium was bombarded with a-particles from a cyclotron.

First, it was hot rolled from k-inch diameter rod to 0.040-inch thick strip, cold rolled to 0.020-inch thick strip, and tensile samples prepared as in the preceding section. Sets of samples were annealed at 925, 1,038, and 1,225C for 1 hour in argon and cooled in the cold zone of the furnace. One set of samples was annealed for 1 hour at 1,038C and water quenched. These samples were then irradiated to approximately 0.1 ppm of helium and tensile tested in the same manner as the 304 stainlesssteel of Part (A).

A non-bombardment control (no He) samples annealed for 1 hour at 1,038C in argon was used as a basis for comparison. These data and data from bombarded samples are presented in Table II.

TABLE I Effect of a-Particle Bombardment on Type 304 Stainless Steel Total Ultimate Elonga- Test Yield Tensile Uniform tion at Temp. Strength Strength Elongation Fracture Sample (C) (1000psi) (l000pai) Control 500 17.0-18.0 68.8-71.6 44.1-45.6 45.4-47.8 unirrudiated 600 12.4-14.4 48.0-52.4 35.5-39.5 40.9-41.9

Irradiated 500 15.0 59.0 43.2 44.3 with 600 15.6 52.0 33.6 42.7 Moderator 700 15.0 31.6 20.6 31.0

irradiated 500 14.4 60.8 44.1 47.5 without 600 12.8 51.8 40.8 432 Moderator 700 14.2 43.0 30.3 41.9

TABLE II Total Elongation of 0.2% Titanium Modified Stainless Steel Test Temperature (C) Sample Condition 500 600 700 800 900 Not bombarded, annealed 1 hr 1038C 33.4 34.4 55.5 55.3 33.7 Bombarded," annealed 1 hr at 1038C, water quenched 32.0 33.7 56.9 65.9 58.4 Bombarded. annealed 1 hr at 925C, slow cooled 33.5 41.4 65.3 84.9 86.1 Bomharded, annealed 1 hr at 1038C, slow cooled 33.1 35.5 63.2 61.9 61.1 Bombarded, annealed 1 hr at 1225C, slow cooled 33.9 31.5 35.4 48.4 38.8

No Helium "Irradiated to 0.1 ppm helium It is noteworthy that the titanium-modified stainless The titanium-doped specimens were then irradiated in steel has higher tensile ductility from 700-900C than a nuclear reactor to a thermal neutron dosage of 1 X the type 304 stainless steel. However, while 0.1 ppm of neutrons/cm and 1.5 X 10 fast neutrons/ 3 at helium causes drastic reduction in the ductility of una temperature of 52C. The results are summarized in modified type 304 stainless steel, the titanium-modified 5 Table IV below.

TABLET/A m Short Time Tensile Ductility of irradiated Austenitic Stainless Steel at 842C Ti Content 0 0.1 0.2 0.3 0.4 0.6 1.0 1.2

% Elongation 12 3s 60 34 19 i6 13 12 steel annealed an hour at l,038C undergoes'no ex- The data shows clearly that titanium additions in the perimentally detectable change in ductility as a result 15 range 0.1-0.30 weight percent have a profound effect of bombardment to 0.1 ppm of helium. on the post-irradiation ductility of austenitic stainless steels. The sharp drop in post-irradiation ductility at ti- EXAMPLE II tanium concentrations beyond 0.30 percent is not unpmcedurahy, the results of this ihvehtioh are derstood completely. It issug gested, however, that the achieved by adjusting the composition of the:meta| or 20. higher concentrations of titanium lead to the format1on alloy during its synthesis while it is still in molten form. I induction of a l h P Before the usual forming, rolling, forging, casting, or f' i alloys cohtalhlhg htahlhm cohcehh'ahoha a other operation, the content of the boride-forming elehlgh as P m a dlspersed intragranular p ment is so adjusted as to bring it within a concentration tatad Phase still have an pp y higher ductility effective to produce the desired intergranular precipithan the Same austehihc Steel exposed to the same tate. Accordingly, a stainless steel composition 0.014% Clear environment but Without the stipulated titanium c, 1.54% Mn, 0.72% Si, 18.31% Cr, 10.0% Ni, addition- 0.0014% B, and the rest Fe was prepared and doped to contain 0.14 weight per cent titanium. The resultant EXAMPLE IV melt casting was then fabricated into rod form and heat A 304 ype Stainless steel having the same general treated for 1 hour in an argon atmosphere at 900C in composition as expressed in Example III except that the one case and at l ,036 C in the other. Tensile specimens carbon concen ration was 0.06 weight per cent was from each heat-treated sample were then measured for doped with 0.2 weight percent titanium and heat ductility, then exposed to neutron irradiation to a total treated in the solid state at a temperature of l,036C neutron fluence of l X 10 nvt at a temperature of for a period of 1 hour to produce an intragranular pre- 52C. The irradiated specimens were measured for cipitated phase. The modified alloy and a control alloy ductility and the results before and after are summa- (without titanium addition) were then exposed to a rized in Table 11] below. total thermal neutron dosage of 10 neutrons/cm at a "w rA Bilaiii i W Pre-irradiation Ductility elongation at 842C) Heat treatment "C unirradiated irradiated (to lXlOnvt) Several important and surprising results were noted. temperature of C. Ductility measurements were Although the pre-irradiation ductility of the specimen taken during the progress of irradiation. The results are heat treated at 1,036C was greater than for the alloy 50 summarized in Table V. given a 900C heat treatment, the reverse was true after TABLE v irradiation. In fact, the ductility of the 900C heattreated specimen was nearly twice that of the 1,036C

Thermal Neutron Total Elongation lb heabtreated specimen. Upon examination of the mihow "1cm, Mame, 33 Unmadmm 8s crostructure of the two alloys, it was found that a fine intragranular dispersion was developed in the 900C :3 g; heat-treated alloy, whereas no such precipitate was I020 46 20 found in the alloy heat treated at the higher tempera- 10" 22 8 ture. It will be seen that the titanium-doped specimen EXAMPLE maintained a considerable percentage of its original A series of melt castings were made up to an austenductility up to a neutron dosage of 10 neutro s/ itic 304 stainless steel type composition (17-20 weight whereas, the undoped stainless steel experienced draspercent chromium, 8-12 weight percent nickel, 0.02 tic losses. Note that at 10 nv the titanium modified weight percent carbon, 5 ppm boron, and the balance specimen had a ductility of more than twice that of the iron). Each melt was doped with increasing amounts of unmodified alloy. At a neutron dosage of 10 nv the titanium and the titanium-doped specimens were then ductility of the titanium modified alloy was nearly three heat treated in the solid state for 1 hour at 1,036C. times that of the unmodified alloy.

EXAMPLE V A nickel-base alloy, Hastelloy N, and an alloy of similar composition but modified to contain 0.52 weight per cent titanium were both heat treated at a temperature of 1,177C for 1 hour and then rapidly cooled to room temperature. Both alloys were then exposed to the radiation of a nuclear reactor to a total thermal neutron dose of 5 X nvt at a temperature of 650C. Tensile specimens of the neutron irradiated alloys were then tested for creep, i.e., stress to rupture at an imposed load of 32,350 psi at a temperature of 650C. The results are summarized in Table V1 below.

TABLE VI 17% Mo, 5% Fe, 0.06% C, 0.6 Si, 0.6 Mn, max. 0.004 B, and the rest Ni "12% Mo, 0.06% C, 0.01 Si, max. 0.001 B, 0.52 Ti, and the rest Ni The improvement in post-irradiation ductility and creep properties effected by the titanium addition and precipitation heat treatment is startling and unexpected. The short term tensile ductility of the modified alloy was increased over an order of magnitude from less than 0.5 to 7.1 percent total elongation. The long term tensile strength or creep (or stress to rupture life) was increased to over 356 hours from a life of less than 10 minutes. Improvements in short time tensile strength, as well as in stress rupture life, have been noted in alloys modified to contain from 0.1-0.8 weight percent titanium. For example, an alloy modified to contain 0.1 weight percent titanium had an elongation of 1.2 and a stress rupture life of 167 hours; an alloy modified to contain 0.4 weight percent titanium had a stress rupture life of570 and an elongation of 5.15; an alloy containing 0.5 weight percent titanium had a rupture life of over 900 hours; at 0.6 weight percent titanium a rupture life of in excess of 600 hours; and at 0.8 percent titanium a rupture life of over 200 hours. Beyond this concentration of titanium, the short and long term tensile strength drops sharply approaching that of the unmodified alloy.

EXAMPLE V1 Hastelloy N specimens (17% Mo, 5% Fe, 0.06% C, 0.6% Si, 0.6% Mn, max. 0.01 B, and the rest Ni) were prepared with varying amounts of zirconium added up to 0.5%. Tensile and creep testing was then undertaken after neutron irradiation to 2.5 X 10 nvt at 650C. The

Tests started at 32,000 psi and ran for 670 hrs. at which time the stress was raised to 40,000 psi and the specimens sustained the stress the additional hours indicated, and then fractured with the indicated elongations.

TABLE V111 ASTM Total Elongation, Grain at 704C at 842C Size Unirradiated Irradiated Unirradiated lrradiated It is seen that the ductility of the irradiated material is a definite function of grain size. In order to control grain size within the context of the present invention, it is a desirable and preferred condition to control the prior fabrication history of the metal or alloy in such a manner as to produce a recrystallization temperature of from l00200C less than the precipitate-inducing heat-treating temperature. In that way, a range of temperature will be available for grain growth to reach the desired range of grain size.

In summary, it can be stated that neutron irradiation at elevated temperatures adversely affects the ability of metals and alloys containing incidental amounts of boron to resist intergranular fracture; and experimental evidence points to the principal cause as one related to the production of helium from the nuclear transmutation of boron-10 with neutrons having thermal energy. A second source is from reaction between major alloy constituents and fast neutrons. The present invention provides at least a partial solution to the problem of irradiation-induced embrittlement by modifications in structure and composition of the alloy that reduces the amount of helium located in the intergranular boundaries of the irradiated alloy. To reduce the amount of helium at the grain boundaries in accordance with this invention, movement of the helium to the grain boundaries is restricted by conversion of the boron to an insoluble phase, desegregated, or redistributed as a fairly fine grained dispersion, with grain sizes in the range ASTM 8-11 generally being preferred. This approach involves the provision of an effective amount of a boride-forming element combined with a heat treatment which induces precipitation of an intragranular dispersion. In accordance with applicants proposed interpre tation, helium sinks are produced within the matrix of the grain which greatly reduce the quantity of helium moving to the intergranular boundaries. The addition of effective quantities of boride-forming elements are thus believed to form complex metal borides dispersed homogeneously within the matrix, producing substitute intragranular interfaces which serve as a depository for helium. While a reducing irradiation embrittlement has been illustrated in two classes of alloys commonly used as structural materials for nuclear reactors (austenitic stainless steels and nickel-base alloys of the Hastelloy N class), the technique for practicing the inventive concept is essentially independent of the initial alloy composition. Thus, post-irradiation ductility of Haste]- loy N alloys comprising the. class of nickel-base alloys containing l-22 weight percent molybdenum, a relatively minor amount of carbon, up to 0.08 percent, 6-8 weight percent chromium, and the balance nickel can be improved by incorporating an effective amount of the selected boride-forming element as an intragranular dispersion formed at a temperature above the intended service condition. It is only assumed that the ductility of the modified unirradiated metal or alloy is of a sufficiently high magnitude to be useful, for it should be understood that the present invention is an improvement in the sense that it retains or conserves the ductility developed by the heat-treated material rather than imparting an absolute increase in ductility.

Having thus described our invention, we claim:

1. A method for imparting radiation resistance to a nickel-base alloy which comprises formulating an alloy consisting essentially of, in weight percent, boron up to 0.01%; 15-22% molybdenum; carbon, when present, up to 0.08%;.6-8% chromium; a boride-forming element selected from the group consisting of titanium from 0.1 0.8 percent and zirconium from 0.1 1.2 percent, said element being present in a concentration sufficient to ensure formation of an intragranular precipitate; and the balance nickel, and then heating said alloy at a temperature of about 650F for a time sufficient to form said intragranular precipitate.

2. An age-hardened alloy consisting essentially of, in weight percent, boron up to 0.01%; l5-22% molybdenum; carbon, when present, up to 0.08%; 68% chr0- mium; a boride-forming element selected from the group consisting of titanium from 0.1 0.8 percent and zirconium from 0.1 1.2 percent; and the balance nickel, said selected element existing at least in part within an intragranular precipitate, said alloy being further characterized in having enhanced post-irradiation tensile properties.

[UNIT D STATES ATENT OFFICE "CERTIFICATE OF CORRECTION Patent No. v 3,804,680 I Dated April 16, 1974 lnvenee e William R. Martin et al.

'It is 'certified that error appears in the above-identified patent and that s aid Letters Patent are hereby corrected as shown below:

Column 2, line '3, "denotion" should read function Column 4, line 4, "sequentially not" should read not essentially e 7 Columns 3 nd 4, Table II, last column, first 1 ine, "33. 7" should read 33.7

Column 6, Table V, the heading "Modified 33" should v read Modified SS the heading "Unmodified 88" should read Unmodified SS v 7 Column 7, Table VI under "Rupture Life", "0.l" should read 0.l under Total Elongation", "0.50" should read 0.50 first footnote, "0.004 B" should read 0.001 B Column l0, line 8, "650F" should read 650C Signed and sealed this 1st day of October 1974.

(SEAL) Attest: V MCOY M. GIBSON JR; c. MARSHALL DANN Attesting Officer Commissioner of Patents FORM 'W USCOMM-DC wave-ps9 Q USe GOVERNMENT PRINTING OFFICE 1 I955 O356-334 

2. An age-hardened alloy consisting essentially of, in weight percent, boron up to 0.01%; 15-22% molybdenum; carbon, When present, up to 0.08%; 6-8% chromium; a boride-forming element selected from the group consisting of titanium from 0.1 - 0.8 percent and zirconium from 0.1 - 1.2 percent; and the balance nickel, said selected element existing at least in part within an intragranular precipitate, said alloy being further characterized in having enhanced post-irradiation tensile properties. 